function getRem(pwidth, prem) {
  var html = document.getElementsByTagName("html")[0];
  var oWidth = document.body.clientWidth || document.documentElement.clientWidth;
  html.style.fontSize = oWidth / pwidth * prem + "px";
}
getRem(750, 100)
window.onresize = function() {
  getRem(750, 100) // 编辑器设置 1rem = 100px
}